Chop the asparagus into 1-inch pieces and blanch for approximately 3 minutes. In 4T of butter, saute the onions. Mix onions and asparagus in large bowl with all other ingredients EXCEPT the rest of the butter and the filo dough. Melt remaining 2 sticks of butter. Thoroughly coat the bottom and sides of a 9x13-inch pan with butter. Layer 8 sheets of filo into prepared pan, brushing butter on each layer. Pour the asparagus filling mixture into the pan and top with remaining 8 sheets of filo dough, brushing each with butter as before. Trim the edges and bake for 45 minutes or until golden brown.